Hitherto, the anterior and posterior surfaces of an ophthalmic spectacle lens have either both been of a spherical form or one of the surfaces has been made spherical and the other toroidal if a vision defect known as astigmatism was to be corrected for. The particular radii for the surfaces have been selected by the lens manufacturers so as to yield the best compromise between optical errors in the margins of the lens, lens weight and the flat plate thickness of the lens, and such lenses are known as "Best Form" lenses.